To give those skilled in the art an appreciation for the advantages of the present invention, it is necessary to understand the context in which the invention will be used. Since this invention will be used in communication satellite payloads and its use will require a departure from conventional communication satellite design, a brief summary of prior art in satellite payload design will be given to provide an understanding of the numerous advantages obtained through the use of the present invention.
Communication satellites employ payloads that all operate in the same basic fashion. A signal received with a receive antenna is passed through a repeater and then transmitted with a transmit antenna. The receive antenna serves to discriminate which directions the receive signal power will be admitted. The transmit antenna serves to discriminate which directions the transmit signal power will be directed. The directional properties of transmit and receive antennas are characterized by their antenna directivity patterns. The repeater performs several basic functions as follows:
1) It performs low noise amplification of the received signal and filters out signals not in the receive band. PA0 2) It translates the signal from the receive frequency to the transmit frequency and filters out signals not in the transmit band. This is done to prevent transmit signal feedback from corrupting the received signal. PA0 3) It amplifies the signal to the required output power level to close the communication link. PA0 1) Each element amplifier sees all carriers in a signal; consequently, the amplifier needs to be operated in a more linear region resulting in amplifier power conversion efficiency dropping to the 20% range as mentioned above. PA0 2) Getting rid of waste heat is complicated by the low power conversion efficiency and the orientation of the array.
Commercial communication satellite payloads generally consist of reflector antennas and channelized repeaters. This type of payload is selected because it allows DC power supplied by the spacecraft to be efficiently converted into radiated microwave signal power with the proper antenna directivity pattern. Reflector antennas for transmitting and receiving can produce high gain shaped contour directivity patterns with very little loss. This is particularly true of shaped reflector antennas where the need for a beamforming network and the associated losses are eliminated.
Channelized repeaters have the advantage of efficiently converting DC power supplied by the spacecraft into microwave signal power. This is accomplished in the process of amplifying the signal to the required output power level. Advantage is taken of the fact that the signal is generally composed of individual frequency components known as carriers. Each individual carrier in the signal is filtered out and passed through its own individual channel. Each channel contains an amplifier that is driven into saturation by the carrier in order that DC to microwave power conversion efficiency be maximized. Typically, amplifier power conversion efficiency can be as high as 50% (half of the DC power is converted to microwave signal power) for Traveling Wave Tube Amplifiers (TWTAs) and a little above 40% for Solid State Power Amplifiers (SSPAs) depending on the frequency. The respective amplified carriers from each channel are then filtered and combined in an output multiplexer to form the amplified signal to be passed to the transmit antenna.
It should be noted that if a signal consisting of multiple carriers is used to drive a single amplifier into saturation, the amplified signal would be degraded by intermodulation interference resulting from the nonlinear transfer characteristics of the saturated amplifier. To reduce the intermodulation interference to an acceptable level, the amplifier needs to be operated in a more linear region so the power level of the signal applied to the amplifier has to be reduced 50% to 60%. This results in amplifier power conversion efficiency dropping to the 20% range.
Although conventional payloads consisting of reflector antennas and channelized repeaters have an advantage in power conversion efficiency, there is a major drawback to this type of design. The drawback is flexibility. The directivity pattern of a reflector antenna is determined by the physical construction of the feed array and beamforming network or the shape of the reflector surface. These attributes are not easily changed particularly in orbit. The output multiplexer of a channelized repeater is required to have very low loss; consequently, it is constructed of waveguide filters and couplers. Once the repeater is constructed, the frequency allocations of the individual carriers can not be changed. The result is a relatively expensive custom designed spacecraft that has limited value for missions other than the one it was specifically designed for.
For many years it has been known that phased array antennas can provide the flexibility of electronic control of antenna directivity pattern shape and position. A phased array is a collection of many antennas or antenna elements that radiate individual coherent signals that are phase and amplitude weighted to provide constructive interference in some directions and destructive interference in other directions. The directional properties of the constructive and destructive interference, characterized by the antenna directivity pattern, can be modified by changing the amplitude and phase weighting of the antenna elements. The antenna element weighting is accomplished in the beamforming network.
Earlier phased array designs used passive phase shifting and power dividing components employing ferrite to control the weighting of the antenna elements. No signal amplification occurred in the antenna elements or beamforming network. This architecture, generally referred to as a passive phased array, provided directivity pattern flexibility but had the disadvantage of being heavy and expensive since the beamforming network needed to be made of metallic waveguide components to minimize loss. For commercial communication satellite applications, where low weight and low loss are of the utmost importance, the weight and loss of the passive phased array proved to be much higher than conventional designs and consequently the passive phased array never really caught on.
More recent phased array work has involved using amplifiers at each antenna element in the array. This type of phased array is generally referred to as an active phased array. An amplifier at each antenna element allows the use of more lossey beamforming network technologies such as microstrip and Monolithic Microwave Integrated Circuit (MMIC) devices for phase shifting and attenuating. This provides the potential to greatly reduce weight, size and cost of the active array. The use of active arrays also allows more lossey repeater technologies such as Surface Acoustic Wave (SAW) devices for filtering and MMICs for signal processing and routing. This eliminates the need for much of the hardware in conventional repeaters such as waveguide multiplexers and filters, high power Traveling Wave Tube Amplifiers (TWTAs) and redundancy rings, and the associated waveguide runs and support structure. The result is large reductions in weight, size and cost of the repeater. However, it should be noted that transmit active phased arrays have two major problems as follows:
As a consequence of low amplifier efficiency, payloads with active phased arrays require more bias power and dissipate more heat than conventional payloads with the same communication specifications. Therefore, a spacecraft with active phased arrays requires a larger heavier power supply subsystem (i.e. larger solar cell arrays, more batteries etc.). For reliability, the junction temperature of each Solid State Power Amplifier in each array element must be maintained below 100.degree. C. and temperature swings should be kept below 50.degree. C. Since there is a larger amount of waste heat to be rejected with active arrays and there is no convection cooling in space, maintaining the proper temperature specifications becomes very difficult. The thermal design is further complicated by the fact that the radiating surface of the array is directed towards the Earth; consequently, the radiating surface of the array is exposed to solar radiation with near normal incidence for arrays in geostationary orbit. Thus, solar heating of the array also becomes a problem.
Linearizing circuits have been used to improve the efficiency of amplifiers used with multicarrier signals and research in this area is the subject of active investigation.
Several solutions to the thermal problems of active arrays have also been proposed. For example, D. Michel, et al in "A Ku-Band Active Antenna Program", AIAA 14th International Communications Satellite Conference, Washington D.C., Mar. 22-26, 1992, pp. 1261-1271, describes one of the more common solutions that employs the use of heat pipes on the back side of the active array to conduct heat to separate thermal radiators on the north and south sides of a body stabilized spacecraft. Solar heating of the radiating surface of the array was minimized by the use of thermal control paints. This design works well and has a lot of heritage but it has the disadvantage of being very heavy. Thermal control paints also degrade relatively quickly.
Radiating heat out of the north and south sides of a body stabilized communication satellite is a standard technique for conventional payloads where all the high power amplifiers are mounted on the inward sides of the north and south thermal radiating panels and heat pipes imbedded in each panel distribute the waste heat uniformly.
A. Molker, in "High-Efficiency Phased Array Antenna for Advanced Multibeam; Multiservice Mobile Communication Satellite", 3rd International Conference on Satellite Systems for Mobile Communications & Navigation, London, England, Jun. 7-9, 1983, pp. 75-77, describes a rather novel technique of attaching silvered second surface mirrors to the bottom of the reflector on a short back-fire antenna element to reject heat and mounting an active array of such elements on the nadir face of a body stabilized spacecraft. This design eliminates the expense and weight of a heat pipe network and the mirrors minimize the effects of solar heating but it can radiate only low thermal power densities (less than 20 Watts per square foot).
Perhaps the most advanced thermal design concept for active phased arrays has come from the spaced based radar field. L. M. Herold, et al in L. J. Cantafio (editor), Spaced Based Radar Handbook, Norwood, Mass.: Artech House, Inc, 1989, pp. 319-348, describes using the active array as both a microwave and thermal radiator like A. Molker but proposes that the active array be constructed as a thin panel structure to allow heat to be radiated out of both sides. Provided that the surface thermal properties are properly designed, relatively large thermal power densities (about 60 Watts per square foot) can be radiated using this concept because at least one of the array sides is not facing the sun at any particular time. No details were disclosed by L. M. Herold, et al about the actual construction of such an array panel.
A pending U.S. patent application entitled "Phased Array Antenna for Efficient Radiation of Microwave and Thermal Energy" by inventor Alan R. Cherrette and assigned to Hughes Aircraft Company on Feb. 26, 1993 discloses a thin light weight active array panel that uses silvered second surface mirrors to form a novel and efficient microwave and thermal radiating surface on one side of the panel and an efficient thermal radiating surface on the opposing side. Use of such active array panels in a communication satellite payload significantly reduces payload weight and cost compared to conventional payload designs. The active array payload weight reduction may also offset the weight increase in the spacecraft power subsystem required to compensate for the low amplifier efficiency discussed earlier. The disclosure above does have a major deficiency however, and that is that only linear polarized microwave power can be produced.
The present invention corrects this deficiency by providing a novel microwave and thermal radiating surface where the microwave power can be produced in any polarization. In fact, with this invention the polarization can even be electronically controlled. These and other features and advantages of the present invention will become apparent from the following descriptions.